1. Field of the Invention
The present invention relates to an energization control device configured to control power applied to a heater, and particularly relates to an image forming apparatus configured to control power applied to a fusing unit used during electrophotography process.
2. Description of the Related Art
A heat roller system has been widely used for an electrophotography copier, as a method of fusing an image on a recording material including a sheet of paper or the like. FIG. 2 is a sectional view of a fusing device using the heat roller system. A toner image formed on a photoconductor is transferred onto one of the faces of a recording sheet P. The above-described toner image is fused onto the recording sheet P by being heated and pressed when the recording sheet P is transmitted between a fuser roll 202 and a pressure roll 203 along the direction of an arrow A.
The fuser roll 202 includes a cylindrical roll 202a and a halogen heater 202b provided in the cylindrical roll 202a as a heat source. In general, the heat roller system allows for performing fusing operations with stability by increasing the heat capacity of a fuser roller and storing heat in the fuser roller. However, due to the increased heat capacity, it takes a long time until the temperature of the fuser roller reaches a desired temperature. Further, the heat roller system consumes power while waiting for image forming operations, so as to keep the fuser roller at a constant temperature.
According to the invention disclosed in U.S. Pat. No. 5,149,941, a film-heating type heating device is used as a system to reduce the above-described waiting time. A film-heating type fusing device uses a plane heater including a ceramic heater or the like (hereinafter referred to as the plane heater) as the heat source.
The fusing device using the above-described ceramic heater performs a fusing operation by directly pressing a film sliding on the heater against a recording sheet. Therefore, the temperature of the heater significantly affects the fusing temperature of the recording sheet. Accordingly, the heater temperature should be stabilized in units of short time, so as to reduce the temperature ripple of the heater. In general, the method of controlling the percentage of power applied to the heater per unit time has been used as a method of controlling the short-time unit.
As a method of controlling the power energization percentage, the following method has been used. Namely, a single half wave of an alternating current voltage (50 Hz) generated by a commercial power supply is determined to be the unit time, and the percentage of power applied during the time period corresponding to twenty half waves (200 ms) or the like is adjusted. The above-described adjusting method is attained by using a power energization pattern table. According to the above-described power energization pattern table, the state in which the power corresponding to all of the twenty half waves is applied to the heater is determined to be 100%. Further, the power energization percentage is changed in steps of 10%. A desired power energization amount is calculated by comparing the detection result of the heater temperature with a target fusing temperature every 200 ms, so as to keep the fusing temperature constant, and a power energization pattern used for the next 200 ms is determined.
However, since the amount of power applied to the heater is changed with relatively high speed, the occurrence of flicker often becomes significant. The flicker denotes the state in which lighting connected to the same line used for the power supply flickers due to fluctuations in a power supply voltage, the fluctuations being caused by power consumption of an electric appliance.
United States Patent Application No. 20060051118 discloses the following invention as a method of reducing the flicker of the film heating system using the above-described resistor heater. Namely, the amounts of power applied to two resistor heaters per unit time are equalized. Further, by making the resistance values of the two resistor heaters different from each other, the difference between the drops of the power supply voltages is reduced, where the drops vary based on the type of the power energization pattern. Further, the invention disclosed in Japanese Patent Laid-Open No. 2002-50450 has proposed the method of assigning a heater control pattern to each of the temperature ranges of the heater and switching over to another heater control pattern in sequence, so as to reduce the fluctuations in the power supply voltage.
In the European Community (EC) market, the amount of the flicker occurrence has been restricted by International Electrotechnical Commission (IEC) standards. According to IEC 61000-3-3 standard, a flicker should be measured through a flicker meter. Further, the flicker value should be expressed as a Pst value, and the expression Pst≦1.00 should hold. Further, the Pst value is calculated based on the power supply voltage-fluctuation amount and the responsivity of the flicker meter attained centering on the frequency of 8.8 Hz. The above-described responsivity corresponds to the standard of the threshold value of flicker perceived by a person. That is to say, as the frequency value nears 8.8 Hz, the easier it becomes for a person to perceive the flicker.
The heating device used for the image forming apparatus consumes a relatively large amount of power, such as 1000 W or around. Further, the power energization amount is changed at regular intervals, so as to control the temperature of a fuser heater, so that the flicker occurs easily.
Usually, power is supplied from a commercial power supply, which is an AC power supply, to the heating device. At that time, the power energization percentage on the positive side should be equal to that on the negative size (symmetry in the positive and negative directions) in the unit time (corresponding to every wave of the AC power supply), so as not to affect the power supply. Further, the power energization pattern is selected for every predetermined unit time, so as to keep the fusing temperature in a predetermined temperature range. Further, according to the power energization pattern, a single positive half wave and a single negative half wave are grouped, so as to switch between the power energization state and the non-power energization state of the heater. Consequently, the symmetry in the positive and negative directions is maintained.
According to the above-described configuration, the frequencies of power applied to the heaters are distributed from a low order to a high order centering on a frequency of 50 Hz, which is the frequency of the commercial power supply. Further, since the heater-power-energization pattern is changed at the time determined based on a plurality of half waves of the commercial power supply, the change is made at a frequency lower than the frequency of 50 Hz. According to United States Patent Application No. 20060051118 and/or Japanese Patent Laid-Open No. 2002-50450, no consideration is given to the flicker sensitivity which is increased due to the above-described change in the power energization pattern, the change being made at the low frequency.